1. Field of the Invention
The present invention pertains to data bus systems for transmitting digital information between a plurality of interconnected and serially arranged terminals, and more particularly it pertains to data buses of the active type wherein each terminal receives data from a data source or from another terminal and transmits or retransmits the data to a different terminal in the system and wherein the survivability of the data bus is enhanced.
2. Description of the Prior Art
Recent developments in data transmission systems involve placing all data onto a single transmission line, or set of transmission lines, and then receiving the data along the line as needed, thereby distributing digital data throughout complex communication systems and over large distances, for example on board a ship. Nonetheless, such recently developed systems are subject to degradation when a multiplicity of faults occurs in the single or set of transmission lines.
There are two basic types of data buses in use in large scale distribution systems at the present time, namely passive buses and active buses. The more typical passive bus systems employ transmitters, or drivers, and receivers that couple passively to a transmission line. An active bus, on the other hand, employs active terminals. Each terminal receives data from another terminal in the system and then retransmits the data or transmits new data to a third terminal in the system. Each unidirectional link between terminals in an active bus system therefore comprises a complete transmission element, whereas in the passive data bus systems, the entire length of transmission line comprises a single bidirectional transmission element. As a consequence of this fundamental difference, a passive data bus system is more vulnerable to reflections than is an active data bus system. Indeed, a passive bus will become totally disabled by a single break in the transmission line which causes reflections to propagate throughout the entire system.
Active data bus systems are most commonly arranged in a closed loop or ring structure. With such a structure, continuity of the loop is an important consideration. A single break in the transmission path, although not totally disabling the system as with a passive bus, will prevent data from traversing the entire loop. It is therefore highly desirable to provide means which will serve to maintain loop continuity in the event of damage to terminals and/or transmission lines. This is especially true of active data buses employed in military applications.
One approach to the problem of survivability of an active data loop has been to employ complementary redundant data paths wherein each adjacent pair of terminals is interconnected by two complementary transmission lines carrying data in opposite directions. One transmission path, designated the A channel, forms a clockwise loop. The other transmission path, designated the B channel, forms a counterclockwise loop. In addition to the clockwise and counterclockwise loops, hybrid-loops utilizing both A and B channels can be formed by utilizing two terminals as "end terminals" which transfer data from the A channel to the B channel and vice-versa.
The use of complementary redundant data paths greatly enhances system survivability by permitting optional data loops to be formed. For example, if one or more casualties occur to the clockwise loop, the terminals can cooperatively switch to the B channel and utilize the counterclockwise loop. Similarly, if one or more casualties occur to the counterclockwise loop, the clockwise loop (A channel) can be utilized. Even in the event of damage to transmission paths of both channels between a single pair of terminals, a hydridloop can be formed which will maintain loop continuity throughout the system. However, multiple casualties affecting both channels at different locations will cause degradation by segmenting the system into smaller hybrid sub-loops that cannot intercommunicate.